455 research outputs found
Open-String Non-Associativity in an R-flux Background
We derive the commutation relations for open-string coordinates on D-branes
in non-geometric background spaces. Starting from D0-branes on a
three-dimensional torus with H-flux, we show that open strings with end points
on D3-branes in a three-dimensional R-flux background exhibit a non-associative
phase-space algebra, which is similar to the non-associative R-flux algebra of
closed strings. Therefore, the effective open-string gauge theory on the
D3-branes is expected to be a non-associative gauge theory. We also point out
differences between the non-associative phase space structure of open and
closed strings in non-geometric backgrounds, which are related to the different
structure of the world-sheet commutators of open and closed strings.Comment: 29 pages; v2: added section 3.5, minor changes; v3: published
version, clarifications added in sections 3 and 4, added appendix B.
Multi-plectoneme phase of double-stranded DNA under torsion
We use the worm-like chain model to study supercoiling of DNA under tension
and torque. The model reproduces experimental data for a much broader range of
forces, salt concentrations and contour lengths than previous approaches. Our
theory shows, for the first time, how the behavior of the system is controlled
by a multi-plectoneme phase in a wide range of parameters. This phase does not
only affect turn-extension curves but also leads to a non-constant torque in
the plectonemic phase. Shortcomings from previous models and inconsistencies
between experimental data are resolved in our theory without the need of
adjustable parameters.Comment: 4 pages, 6 figures, submitted, 2 typo's corrected, one reference
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Canonical density matrix perturbation theory
Density matrix perturbation theory [Niklasson and Challacombe, Phys. Rev.
Lett. 92, 193001 (2004)] is generalized to canonical (NVT) free energy
ensembles in tight-binding, Hartree-Fock or Kohn-Sham density functional
theory. The canonical density matrix perturbation theory can be used to
calculate temperature dependent response properties from the coupled perturbed
self-consistent field equations as in density functional perturbation theory.
The method is well suited to take advantage of sparse matrix algebra to achieve
linear scaling complexity in the computational cost as a function of system
size for sufficiently large non-metallic materials and metals at high
temperatures.Comment: 21 pages, 3 figure
Locally non-geometric fluxes and missing momenta in M-theory
We use exceptional field theory to describe locally non-geometric spaces of M-theory of more than three dimensions. For these spaces, we find a new set of locally non-geometric fluxes which lie in the R-R sector in the weak-coupling limit and can typically be characterised by mixed symmetry tensors. These spaces thus provide new examples of non-geometric backgrounds which go beyond the NS-NS sector of string theory. Starting from twisted tori we construct duality chains that lead to these new non-geometric backgrounds and we show that, just as in the four-dimensional case, there are missing momenta associated to the mixed symmetry tensors
Quantitative determination of the femoral offset templating error in total hip arthroplasty using a new geometric model.
AIMS
Traditionally, total hip arthroplasty (THA) templating has been performed on anteroposterior (AP) pelvis radiographs. Recently, additional AP hip radiographs have been recommended for accurate measurement of the femoral offset (FO). To verify this claim, this study aimed to establish quantitative data of the measurement error of the FO in relation to leg position and X-ray source position using a newly developed geometric model and clinical data.
METHODS
We analyzed the FOs measured on AP hip and pelvis radiographs in a prospective consecutive series of 55 patients undergoing unilateral primary THA for hip osteoarthritis. To determine sample size, a power analysis was performed. Patients' position and X-ray beam setting followed a standardized protocol to achieve reproducible projections. All images were calibrated with the KingMark calibration system. In addition, a geometric model was created to evaluate both the effects of leg position (rotation and abduction/adduction) and the effects of X-ray source position on FO measurement.
RESULTS
The mean FOs measured on AP hip and pelvis radiographs were 38.0 mm (SD 6.4) and 36.6 mm (SD 6.3) (p < 0.001), respectively. Radiological view had a smaller effect on FO measurement than inaccurate leg positioning. The model showed a non-linear relationship between projected FO and femoral neck orientation; at 30° external neck rotation (with reference to the detector plane), a true FO of 40 mm was underestimated by up to 20% (7.8 mm). With a neutral to mild external neck rotation (≤ 15°), the underestimation was less than 7% (2.7 mm). The effect of abduction and adduction was negligible.
CONCLUSION
For routine THA templating, an AP pelvis radiograph remains the gold standard. Only patients with femoral neck malrotation > 15° on the AP pelvis view, e.g. due to external rotation contracture, should receive further imaging. Options include an additional AP hip view with elevation of the entire affected hip to align the femoral neck more parallel to the detector, or a CT scan in more severe cases.Cite this article: Bone Jt Open 2022;3(10):795-803
Semi-classical buckling of stiff polymers
A quantitative theory of the buckling of a worm like chain based on a
semi-classical approximation of the partition function is presented. The
contribution of thermal fluctuations to the force-extension relation that
allows to go beyond the classical Euler buckling is derived in the linear and
non-linear regime as well. It is shown that the thermal fluctuations in the
nonlinear buckling regime increase the end-to-end distance of the semiflexible
rod if it is confined to 2 dimensions as opposed to the 3 dimensional case. Our
approach allows a complete physical understanding of buckling in D=2 and in D=3
below and above the Euler transition.Comment: Revtex, 17 pages, 4 figure
Энергетика Австралии
Реформы энергетической отрасли нашей страны вызывают очень много споров и разногласий. Уход от плановой экономике к рыночной не мог не воздействовать и на энергетический сектор. Вопрос о том что лучше госрегулирование или рыночные отношение в данной системообразующей отрасли и лежит в основе данной статьи. Пример Австралии, нам показывает, как можно построить структуру управления энергосистемой в рыночных условиях
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